1. Field of the Invention
This invention relates generally to the field of high speed packetlike data communication between interconnected electronic modules such as, for example, circuit cards within a single card cage. More particularly, this invention relates to a method and apparatus for providing for transport of high speed ATM (Asynchronous Transfer Mode) data cells (packets) between such modules without requiring a master module.
2. Background of the Invention
With the evolution of modern data communication equipment and increased complexity of data communication networks, large numbers of rack or card cage mounted data communications circuits such as modems, DSU's (Digital Service Units), multiplexers and the like frequently require the ability to communicate with one another and transport data to and from one another. To avoid large arrays of complex and expensive cable interconnection, such devices are sometimes interconnected via a common backplane communication bus.
Unfortunately, when speeds on the bus approach several hundred megabits per second, the use of conventional time multiplexing techniques becomes impractical due to difficulties involved with configuration and maintenance of time slot lists associated with the specific connections as well as synchronization of these lists on modules throughout the system. Similarly, conventional packet based systems require an undue amount of overhead and some type of contention mechanism or deterministic slot assignment is required to allow orderly access to the bus while providing load balancing.
Numerous protocols have been developed for use in local and wide area networks to provide similar load balancing and slot allocation functions. One such system is described in Computer Networks, by Andrew Tanenbaum, 1981, Prentice-Hall, Inc., pp 296-307. This protocol is described as a bit mapped protocol. In it, N sequential time slots are allocated for contention by each of N stations on the local area network. Those stations which indicate a need to transmit are assigned in sequential order a time according to their station number in which to transmit a packet. Due to the serial nature of this protocol, its direct use in a module intgerconnecting backplane environment would involve substantial overhead in order to provide inband framing of the data so that stations could recognize the start and end of data. In addition, this publication details no method for determining when a reservation occurs or how to add or delete stations. Also, the protocol apparently requires some form of master station to initiate the reservation system during initialization.
In "The QPSX Man", by Newman et al., IEEE Communications Magazine, Apr. 1988, Vol. 26, No. 4, pp. 20-28, a Queued packet and synchronous switch Metropolitan area network is described. In this system, packets are continuously circulated on the network and when a station wishes to transmit, it finds a packet and marks it for use. The next time the packet comes around, the station places data in the packet. This is accomplished by counting the number of empty packets that pass the station going away from the master station. When the station wishes to transmit a packet, it freezes the empty packet counter and reserves a transmission spot. The station then counts down from this number, the number of empty packets that pass the station going toward the master station. When this count equals zero, the station transmits the packet.
In U.S. Pat. No. 4,700,341 to Huang, which is incorporated by reference, a plurality of positions for packets are provided in a fixed order. The beginning of a packet is signaled by a flag pattern. If a packet is not filled with information, the remainder of the packet is not transmitted but rather is immediately followed by the flag pattern for the next packet position.
The present invention utilizes a parallel bus structure, which provides for an out of band reservation signal, for transporting high speed data over a backplane bus interconnecting electronic circuit modules without need for a master to control such communication.